Ferroelectric thin-film laminated substrate, ferroelectric thin-film device,and manufacturing method of ferroelectric thin-film laminated substrate

ABSTRACT

There is provided a ferroelectric thin-film laminated substrate, including a substrate, and further including a lower electrode layer, a ferroelectric thin-film layer, an upper electrode adhesive layer, and an upper electrode layer being sequentially stacked on the substrate, in which: the lower electrode layer is made of platinum or a platinum alloy; the ferroelectric thin-film layer is made of a sodium potassium niobate (typical chemical formula of (K 1-x Na x )NbO 3 , 0.4≤x≤0.7); the upper electrode layer is made of gold; the upper electrode adhesive layer is made of a metal that has less oxidizability than titanium and can make a solid solution alloy without generating an intermetallic compound with gold; and a part of the upper electrode adhesive layer and a part of the upper electrode layer are alloyed.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a ferroelectric thin-film device, andmore particularly to a ferroelectric thin-film laminated substratehaving lead-free niobate-based ferroelectrics, a ferroelectric thin-filmdevice cut out of the thin-film laminated substrate, and a method formanufacturing the thin-film laminated substrate.

DESCRIPTION OF BACKGROUND ART

Ferroelectrics are very attractive substances because of their peculiarcharacteristics (such as very high relative permittivity, and goodpyroelectric, piezoelectric and ferroelectric properties). So, variousdevices (such as ceramic multilayer capacitors, pyroelectric devices,piezoelectric devices and ferroelectric memories) have been developedand put into use utilizing such peculiar properties. Typicalferroelectrics are perovskite materials such as barium titanate (BaTiO₃)and lead zirconate titanate (Pb(Zr_(1-x)Ti_(x))O₃, PZT). Of these, leadzirconate titanates (PZTs) provide relatively excellent polarization andpiezoelectric properties and are therefore most widely used.

Lead-containing PZTs are specified hazardous substances. However,because there are currently no suitable commercially availablealternative pyroelectric or piezoelectric materials, PZTs are exemptfrom the RoHS directive (the directive on the restriction of the use ofspecified hazardous substances in electrical and electronic equipmentenforced by the European Union and Council of Europe). However, with thegrowing worldwide responsibility towards global environmentconservation, a strong demand exists for development of pyroelectric andpiezoelectric devices using lead-free ferroelectric materials.

Also, with the recent trend toward smaller and lighter electronicdevices, there is an increasing need for ferroelectric thin-film devicesin which a thin-film technology is utilized.

Herein, pyroelectric and piezoelectric thin-film devices will bedescribed below as representatives of such ferroelectric thin-filmdevices. Piezoelectric devices utilize the piezoelectric effect of aferroelectric material, and are widely used as functional devices suchas actuators and stress sensors. Actuators generate a displacement orvibration in response to an applied voltage to a ferroelectric(piezoelectric) material. Stress sensors generate a voltage in responseto a strain produced in a piezoelectric material. Pyroelectric devicesdetect light (including infrared light) utilizing the pyroelectriceffect of a ferroelectric material, and are widely used as infraredhuman body sensors, etc.

Examples of piezoelectric devices utilizing a lead-free piezoelectricmaterial are described below. Patent Literature 1 discloses apiezoelectric thin-film device including, on a substrate, a lowerelectrode, a piezoelectric thin-film and an upper electrode. Thepiezoelectric thin-film is made of an alkali niobate-based perovskitedielectric material of a chemical formula (Na_(x)K_(y)Li_(z))NbO₃ (where0<x<1, 0<y<1, 0≤z<1, and x+y+z=1). A buffer layer of a perovskitecrystal structure material is formed between the piezoelectric thin-filmand the lower electrode. The perovskite buffer layer is highlypreferentially (001), (100), (010) or (111) oriented. According to thisPatent Literature 1, the piezoelectric thin-film device utilizing thelead-free lithium potassium sodium niobate thin-film exhibits sufficientpiezoelectric properties.

The practical use of the lead-free piezoelectric thin-film devicerequires reliability (durability) in long-term use in addition toimprovement in piezoelectric properties. Various techniques intended forthat have been developed.

For example, Patent Literature 2 discloses a piezoelectric film elementincluding: a substrate; a lower electrode structure provided on thesubstrate; a KNN piezoelectric film provided on the lower electrodestructure; at least one diffusion prevention structure provided on theKNN piezoelectric film; and an upper electrode structure provided on thediffusion prevention structure. The upper electrode structure includesan upper electrode made of an Au layer. The diffusion preventionstructure includes, in order from the KNN piezoelectric film, anintermediate adhesive layer and an Au diffusion prevention layer that isprovided on the intermediate adhesive layer and prevents diffusion of Aufrom the upper electrode to the KNN piezoelectric film.

Patent Literature 2 also discloses that the Au diffusion preventionlayer is made of a Pt layer or a Pt alloy layer and that theintermediate adhesive layer is made of a Cr layer, a Ta layer, a Tilayer, a Ti oxide layer, or a Ti nitride layer. According to PatentLiterature 2, a practical piezoelectric film device in which theelectric insulation of the KNN piezoelectric film hardly deteriorates inlong-term use and a piezoelectric film apparatus including thepiezoelectric film device can be provided.

CITATION LIST Patent Literature

Patent Literature 1: JP 2007-019302 A

Patent Literature 2: JP 2014-060267 A

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

As mentioned above, niobate-system ferroelectrics (such as a potassiumsodium niobate, typical chemical formula of (K_(1-x)Nax)NbO₃) are amongparticularly promising materials as lead-free ferroelectrics. However,because niobate-based ferroelectric materials are relatively newmaterials, lamination structures of thin-film devices and manufacturingprocesses thereof are still under development. In order to put suchthin-film devices using niobate-based ferroelectric materials asalternatives for PZT thin-film devices to practical use and massproduction, it is very important to develop and establish the laminationstructures and manufacturing processes of the thin-film devicessatisfying various requirements for the practical use and massproduction.

For example, it is often the case that wire bonding is used to makeelectric connection in incorporating an electronic component intoelectronic equipment, and an indicator of the bondability of wirebonding is bond strength (tensile strength) between a bonding wire andan electrode pad. The piezoelectric thin-film device of PatentLiterature 2 uses an upper electrode made of a gold layer that issuitable for wire bonding from a standpoint of bondability with respectto a bonding wire (e.g., a gold bonding wire). However, from a massproductivity standpoint, a bond strength higher than before (e.g., atensile strength of 60 mN or more) is recently required. Thus, it hasbeen difficult for the technique of Patent Literature 2 to meet suchneed.

In view of the foregoing, it is an objective of the present invention toprovide a niobate-based ferroelectric thin-film laminated substrate thatenables a bond strength higher than before when wire bonding isperformed on a thin-film device cut out of the thin-film laminatedsubstrate, a ferroelectric thin-film device cut out of the thin-filmlaminated substrate, and a method for manufacturing the thin-filmlaminated substrate.

Solution to Problems

(I) According to one aspect of the present invention, there is provideda ferroelectric thin-film laminated substrate, including a substrate,and further including a lower electrode layer, a ferroelectric thin-filmlayer, an upper electrode adhesive layer, and an upper electrode layer,which are sequentially stacked on the substrate. In the ferroelectricthin-film laminated substrate, the lower electrode layer is made ofplatinum (Pt) or a Pt alloy; the ferroelectric thin-film layer is madeof a sodium potassium niobate (typical chemical formula of(K_(1-x)Na_(x))NbO₃, 0.4≤x≤0.7); the upper electrode layer is made ofgold (Au); the upper electrode adhesive layer is made of a metal whichhas less oxidizability than titanium and can make a solid solution alloywithout generating an intermetallic compound with gold; and a part ofthe upper electrode adhesive layer and a part of the upper electrodelayer are alloyed.

In the above aspect (I) of the invention, the following modificationsand changes can be made.

(i) The upper electrode adhesive layer may be made of one selected fromnickel (Ni), cobalt (Co), tungsten (W), and molybdenum (Mo).

(ii) The upper electrode adhesive layer may have a thickness of 5 nm ormore but 50 nm or less.

(iii) A lower electrode adhesive layer may be further stacked betweenthe substrate and the lower electrode layer, the lower electrodeadhesive layer being made of titanium (Ti) and/or a Ti oxide.

(iv) The lower electrode layer may have a main surface with a (111)crystal plane preferential orientation; and the ferroelectric thin-filmlayer may have a crystal system of a pseudo cubic system or a tetragonalsystem, and a main surface with a (011) crystal plane preferentialorientation.

(v) The substrate may be a silicon substrate having a thermally oxidizedfilm on its surface.

(II) According to another aspect of the invention, there is provided aferroelectric thin-film device characterized by using the ferroelectricthin-film laminated substrate of the invention described above.

(III) According to still another aspect of the invention, there isprovided a manufacturing method of the ferroelectric thin-film laminatedsubstrate of the invention described above. The manufacturing methodincludes: a lower electrode layer formation step of forming the lowerelectrode layer on the substrate; a ferroelectric thin-film layerformation step of forming the ferroelectric thin-film layer on the lowerelectrode layer; an upper electrode adhesive layer formation step offorming the upper electrode adhesive layer on the ferroelectricthin-film layer; and an upper electrode layer formation step of formingthe upper electrode layer on the upper electrode adhesive layer. Theupper electrode layer formation step includes a process of depositingthe upper electrode layer under a temperature environment of 50° C. ormore but 200° C. or less to alloy a part of the upper electrode adhesivelayer with a part of the upper electrode layer.

Advantages of the Invention

According to the present invention, it is possible to provide aniobate-based ferroelectric thin-film laminated substrate that enables abond strength higher than before when wire bonding is performed on athin-film device cut out of the thin-film laminated substrate, aferroelectric thin-film device cut out of the thin-film laminatedsubstrate, and a method for manufacturing the thin-film laminatedsubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration showing a cross-sectional view of aferroelectric thin-film laminated substrate according to the presentinvention; and

FIG. 2 is a graph showing exemplary relationships between polarizationvalue and applied voltage with respect to KNN thin-film devices ofComparative Example 2 and Examples 1 and 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present inventors have focused on a sodium potassium niobate(typical chemical formula of (K_(1-x)Na_(x))NbO₃, 0.4≤x≤0.7; hereinafterreferred to as KNN) as a lead-free ferroelectric material that can beexpected to exhibit a ferroelectricity comparable to that of a PZT(typical chemical formula: Pb(Zr_(1-x)Ti_(x))O₃), and have conductedresearch and development with an aim to put KNN thin-film devices intopractical use. In the research and development, the inventors carriedout intensive investigations on a laminated structure of the KNNthin-film device (particularly, the laminated structure of an upperelectrode) that enables a bond strength higher than before when wirebonding is performed on the KNN thin-film device, and on a process ofmanufacturing the laminated structure of KNN thin-film device.

As a result, it is found that the above can be achieved by using, as anupper electrode intermediate layer formed between the ferroelectricthin-film layer and the upper electrode layer, a metal that has lessoxidizability than Ti (means a metal that hardly oxidizes, indicating ametal having a larger ionization potential) and can make a solidsolution alloy without generating an intermetallic compound with Au. Thepresent invention has been made on the basis of this new finding.

Preferred embodiments of the invention will be described below withreference to the accompanying drawings. However, the invention is notlimited to the specific embodiments described below, but variouscombinations and modifications are possible without departing from thespirit and scope of the invention.

FIG. 1 is a schematic illustration showing a cross-sectional view of aferroelectric thin-film laminated substrate according to the invention.As illustrated in FIG. 1, a ferroelectric thin-film laminated substrate10 of the invention has a structure in which on a substrate 1, a lowerelectrode adhesive layer 2, a lower electrode layer 3, a ferroelectricthin-film layer 4, an upper electrode adhesive layer 5, and an upperelectrode layer 6 are stacked in this order. The ferroelectric thin-filmdevice according to the invention can be obtained as a chip with adesired shape by cutting out of the ferroelectric thin-film laminatedsubstrate 10 of the invention. Meanwhile, the upper electrode adhesivelayer 5 and the upper electrode layer 6 may be formed at the stage ofthe ferroelectric thin-film laminated substrate 10 or may be formedafter the device is cut out into a desired shaped chip.

A detailed description is given in accordance with a procedure ofmanufacturing the ferroelectric thin-film laminated substrate 10.

First, a substrate 1 is prepared. A material of the substrate 1 is notparticularly limited, and may be properly selected based on applicationsof the ferroelectric thin-film devices. For example, silicon (Si), SOI(Silicon on Insulator), quartz glass, gallium arsenide (GaAs), galliumnitride (GaN), sapphire (Al₂O₃), magnesium oxide (MgO), zinc oxide(ZnO), and strontium titanate (SrTiO₃) may be used. From the view pointof cost, using Si substrates is preferable among these materials. Whenan electrically conductive material is used as the substrate 1, itssurface is preferably covered with an electrical insulating film (e.g.an oxide film). There is no particular limitation on a method of formingthe oxide film. For example, thermal oxidation and chemical vapordeposition (CVD) are suitable.

Next, the lower electrode adhesive layer 2 is formed on the substrate 1.Although the formation of the lower electrode adhesive layer 2 is notessential, the lower electrode adhesive layer 2 is preferably formed inorder to increase adhesiveness between the substrate 1 and the lowerelectrode layer 3. Furthermore, it is preferable in terms ofadhesiveness or environmental resistance that the material of the lowerelectrode adhesive layer 2 is, e.g., Ti and/or a Ti oxide having athickness of 1 to 3 nm. Ti is an easy oxidability metal (meaning a metalthat oxidizes easily; a metal having a small ionization potential, e.g.,first ionization potential), which tends to bind to an oxygen atompresent on a surface of the substrate 1 into the form of a titaniumoxide (atomic bond state), so that high adhesiveness can be secured.There is no particular limitation on a method of forming the lowerelectrode adhesive layer 2. For example, the lower electrode adhesivelayer 2 is formed preferably by a physical vapor deposition method suchas sputtering, thermal evaporation, and electron beam evaporation.

Next, a lower electrode layer 3 is formed on the lower electrodeadhesive layer 2. As a material of the lower electrode layer 3, Pt or aPt alloy (alloy containing mainly platinum) is preferable. The lowerelectrode layer 3 is an underlayer of a ferroelectric thin-film layer 4,and then in order to improve crystal orientation of the ferroelectricthin-film layer 4, the lower electrode layer 3 preferably has a mainsurface with one of a (001) crystal plane, a (110) crystal plane, and a(111) crystal plane preferential orientation. Furthermore, the lowerelectrode layer 3 preferably has a crystal texture consisting ofcolumnar crystal grains. There is no particular limitation on a methodfor forming the lower electrode layer 3. For example, the lowerelectrode layer 3 is formed preferably by a physical vapor depositionmethod such as sputtering, thermal evaporation, and electron beamevaporation.

Next, the ferroelectric thin-film layer 4 is formed on the lowerelectrode layer 3. In the present invention, a lead-free ferroelectricof a sodium potassium niobate (typical chemical formula of(K_(1-x)Na_(x))NbO₃, 0.4≤x≤0.7; referred to as KNN) is preferable as amaterial of the ferroelectric thin-film layer 4. In order to achievesufficient ferroelectric performance, a crystal system of theferroelectric thin-film layer 4 is preferably a pseudo cubic system or atetragonal system; and the ferroelectric thin-film layer 4 preferablyhas a main surface with a (001) crystal plane preferential orientation.

There is no particular limitation on a method for forming theferroelectric thin-film layer 4 (KNN thin-film) as long as a desired KNNthin-film layer is obtained. Preferable formation methods includesputtering using a sintered body target having a desired chemicalcomposition, electron beam evaporation, and pulsed laser deposition.These formation methods have advantages of excellent chemicalcomposition controllability and crystal orientation controllability ofthe KNN crystal.

The KNN thin-film layer 4 may contain one or more selected from lithium(Li), tantalum (Ta), antimony (Sb), calcium (Ca), copper (Cu), barium(Ba) and Ti in a total amount of 5 atomic percent or less.

Next, an upper electrode adhesive layer 5 is formed on the ferroelectricthin-film layer 4. It is important that the upper electrode adhesivelayer 5 plays a role of increasing adhesiveness between theferroelectric thin-film layer 4 and the upper electrode layer 6 (role ofan adhesive layer), and does not have adverse influences on theperformance and property (e.g., ferroelectricity) of the thin-filmdevice. The present invention is best characterized by a combination ofthe upper electrode adhesive layer 5 and the upper electrode layer 6.

The material of the upper electrode adhesive layer 5 is preferably ametal that has less oxidizability (meaning a metal that hardly oxidizes;a metal having a larger ionization potential, e.g., first ionizationpotential) than Ti and can make a solid solution alloy withoutgenerating an intermetallic compound with Au. For example, one selectedfrom Ni, Co, W and Mo is preferably used. These metals are moreresistant to oxidation than Ti. Therefore, the percentage (probability)of generating an oxide during deposition is sufficiently small. Forexample, it is considered that an atomic bond state with respect tooxygen is generated only in an interface region between theferroelectric thin-film layer 4 and the upper electrode adhesive layer5.

In addition, the aforementioned metals (Ni, Co, W and Mo) can be a solidsolution alloy without generating an intermetallic compound with Au.Therefore, it is considered that during deposition of an upper electrodelayer in a subsequent process, the metals are alloyed with a part of Auof the upper electrode layer 6 to generate a solid solution alloy.Because an intermetallic compound is not generated, the origin ofbreakage or fracture is hardly generated, and the solid solution enablesmixing at atomic level. As a result, it is considered that highintegrity and adhesiveness can be obtained, so that a high bond strengthcan be obtained.

The upper electrode adhesive layer 5 preferably has a thickness (averagefilm thickness) of 5 nm or more but 50 nm or less. When the thickness ofthe upper electrode adhesive layer 5 is less than 5 nm, an alloyingreaction with Au of the upper electrode layer 6 is insufficient(shortage of the amount of solid solution alloy to be generated),resulting in an insufficient effect of improving bond strength. When thethickness of the upper electrode adhesive layer 5 exceeds 50 nm, theeffect of improving bond strength is saturated, and the processing costof micro fabrication (e.g., dry etching) of the upper electrode adhesivelayer 5 and the upper electrode layer 6 increases.

Next, the upper electrode layer 6 is formed on the upper electrodeadhesive layer 5. According to the invention, Au is preferably used as amaterial of the upper electrode layer 6 from a standpoint of the bondstrength of wire bonding (bondability to the bonding wire). There isalso no particular limitation on a method of forming the upper electrodelayer 6. Similarly to the case of the lower electrode layer 3, aphysical vapor deposition method (e.g., sputtering, thermal evaporation,and electron beam evaporation) may be used preferably.

The deposition of the upper electrode layer 6 is preferably carried outunder a temperature condition at 50° C. or more but 200° C. or less,more preferably at 100° C. or more but 200° C. or less. Depositing theupper electrode layer 6 at a predetermined temperature enablesacceleration of an alloying reaction (generation of a solid solutionalloy) between a part of the upper electrode adhesive layer 5 and a partof the upper electrode layer 6. When the temperature during depositionis less than 50° C., the alloying reaction is insufficient. When thetemperature during deposition exceeds 200° C., the metal of the upperelectrode adhesive layer 5 is prone to be oxidized, preventing thealloying reaction. In addition, excessive oxidization of the upperelectrode adhesive layer 5 is highly prone to lead to deficiency ofoxygen in the ferroelectric thin-film layer 4, which can be a cause ofdegradation of the ferroelectricity.

EXAMPLES

The present invention will be described more specifically below by wayof examples. However, the invention is not limited to the specificexamples below.

[Preparation of Ferroelectric Thin-Film Laminated Substrate]

On the basis of the thin-film laminated structure (substrate 1, lowerelectrode adhesive layer 2, lower electrode layer 3, ferroelectricthin-film layer 4, upper electrode adhesive layer 5, upper electrodelayer 6) illustrated in FIG. 1, several kinds of ferroelectric thin-filmlaminated substrates having a different combination of the upperelectrode adhesive layer 5 and the upper electrode layer 6 wereproduced. As the substrate 1, an Si substrate with a thermal oxide film(four-inch wafer having a (100) plane orientation, a wafer thickness of0.525 mm, a thermal oxide film thickness of 200 nm, a substrate surfaceroughness Ra=0.86 nm) was used.

In the film formation steps below, the thickness of each layer (lowerelectrode adhesive layer 2, lower electrode layer 3, ferroelectricthin-film layer 4, upper electrode adhesive layer 5, upper electrodelayer 6) was controlled by controlling the film formation time based onthe film formation rate determined in advance. Also, the thicknessmeasurement for calculation of each film formation rate was conducted bythe X-ray reflectivity technique using an X-ray diffractometer (X'PertPRO MRD, available from PANalytical B.V., Spectris Co., Ltd.).

First, a Ti layer having a thickness of 2.2 nm was deposited on the Sisubstrate 1 by RF magnetron sputtering as the lower electrode adhesivelayer 2 to enhance adhesion between the substrate 1 and the lowerelectrode layer 3. Subsequently, a Pt layer having a thickness of 205 nmwas deposited by RF magnetron sputtering as the lower electrode layer 3on the Ti lower electrode adhesive layer 2. The sputtering conditionsfor the lower electrode adhesive layer 2 and the lower electrode layer 3were as follows: targets of pure Ti (for the Ti adhesive layer) and purePt (for the Pt electrode layer); discharge power of 200 W; sputteringgas of Ar; and gas pressure of 2.5 Pa. The sputtering was carried outusing a RF sputtering machine (SH-350-T10, available from ULVAC, Inc.)(the same machine was used in all the remaining sputtering processesdescribed below). The lower electrode layer 3 was subjected to thesurface roughness measurement and observed to have an arithmetic meanroughness Ra of 0.86 nm or less.

After the formation of the Pt lower electrode layer 3, a KNN((K_(0.35)Na_(0.65))NbO₃) thin-film having a thickness of 1.9 μm wasdeposited by RF magnetron sputtering as the ferroelectric thin-filmlayer 4 on the lower electrode layer 3. The sputtering condition for theKNN thin-film 4 was as follows: target of sintered(K_(0.35)Na_(0.65))NbO₃; substrate temperature of 400 to 600° C.;discharge power of 700 to 800 W; sputtering gas of O₂/Ar mixture (O₂/Arratio=0.005); and gas pressure of 0.3 to 1.3 Pa.

(Evaluation of Crystal Systems of KNN Thin-Film)

Normally, perovskite KNN crystals have a tetragonal structure in whichthe c-axis is longer than the a-axis (c/a>1). That is, when formed KNNcrystals satisfy the crystalline condition of “c/a>1”, the crystals aremore stable and the crystallinity thereof is high. When an electricfield is applied along the c-axis of a perovskite-type ferroelectriccrystal with a small initial strain, a larger polarization (and thus ahigher gain in piezoelectric or ferroelectric performance) is obtained.

On the other hand, unlike bulk crystals, thin-film crystals formed on asubstrate are prone to distortion caused by the influence of thesubstrate or the underlayer. Specifically, the KNN thin-film formed on asubstrate may have a pseudo cubic crystal system with “c/a≤1” (hereinmeaning “a crystal system closer to a cubic system than to a propertetragonal system”) or a tetragonal crystal system with “c/a>1” (hereinmeaning “a crystal system closer to a proper tetragonal system”).Therefore, the crystal system of each of the KNN thin-films formed abovewas evaluated by X-ray diffraction (XRD). The results showed that theKNN thin-film laminated substrates, each of which was provided with theKNN thin-film mainly having a tetragonal system with “c/a>1”, wereobtained.

Preparation of Comparative Example 1

A Ti layer having a thickness of 10 nm was deposited as the upperelectrode adhesive layer 5 on the KNN thin-film layer 4 by electron beamevaporation. Subsequently, an Au layer having a thickness of 300 nm wasdeposited as the upper electrode layer 6 on the Ti upper electrodeadhesive layer 5 by electron beam evaporation. The depositions of theupper electrode adhesive layer 5 and the upper electrode layer 6 (by theelectron beam evaporation) of Comparative Example 1 were carried outwithout substrate heating. Thus, a KNN thin-film laminated substrate ofComparative Example 1 was produced. The electron beam evaporation wascarried out using an electron beam evaporation machine (EX-400-008,available from ULVAC, Inc.) (the same machine was used in all theremaining electron beam evaporation processes described below).

Preparation of Comparative Example 2

Another KNN thin-film laminated substrate as Comparative Example 2 wasproduced in the same manner as Comparative Example 1 described above,except that the depositions (electron beam evaporation) of the upperelectrode adhesive layer 5 and the upper electrode layer 6 were carriedout under the condition where a substrate temperature was 100° C.

Preparation of Comparative Example 3

Another KNN thin-film laminated substrate as Comparative Example 3 wasproduced in the same manner as Comparative Example 2 described above,except that a Cr layer having a thickness of 10 nm was deposited as theupper electrode adhesive layer 5 on the KNN thin-film layer 4 byelectron beam evaporation (substrate temperature of 100° C.)

Preparation of Comparative Example 4

Another KNN thin-film laminated substrate as Comparative Example 4 wasproduced in the same manner as Comparative Example 2 described above,except that an Ni layer having a thickness of 2 nm was deposited as theupper electrode adhesive layer 5 on the KNN thin-film layer 4 byelectron beam evaporation (substrate temperature of 100° C.).

Preparation of Example 1

Another KNN thin-film laminated substrate as Example 1 was produced inthe same manner as Comparative Example 2 described above, except that anNi layer having a thickness of 10 nm was deposited as the upperelectrode adhesive layer 5 on the KNN thin-film layer 4 by electron beamevaporation (substrate temperature of 100° C.).

Preparation of Example 2

Another KNN thin-film laminated substrate as Example 2 was produced inthe same manner as Comparative Example 2 described above, except that anNi layer having a thickness of 50 nm was deposited as the upperelectrode adhesive layer 5 on the KNN thin-film layer 4 by electron beamevaporation (substrate temperature of 100° C.).

Preparation of Examples 3 to 5

Another KNN thin-film laminated substrate as Example 3 was produced inthe same manner as Comparative Example 2 described above, except that aCo layer having a thickness of 10 nm was deposited as the upperelectrode adhesive layer 5 by electron beam evaporation (substratetemperature of 100° C.). Another KNN thin-film laminated substrate asExample 4 was produced in the same manner as Comparative Example 2described above, except that a W layer having a thickness of 10 nm wasdeposited as the upper electrode adhesive layer 5 by electron beamevaporation (substrate temperature of 100° C.). In addition, another KNNthin-film laminated substrate as Example 5 was produced in the samemanner as Comparative Example 2 described above, except that an Mo layerhaving a thickness of 10 nm was deposited as the upper electrodeadhesive layer 5 by electron beam evaporation (substrate temperature of100° C.)

[Preparation of Ferroelectric Thin-Film Device]

Small pieces (20 mm×20 mm) were cut out of the KNN thin-film laminatedsubstrates of the aforementioned Comparative Examples 1 to 4 andExamples 1 to 5 to produce measurement/evaluation KNN thin-film devicesof Comparative Examples 1 to 4 and Examples 1 to 5.

[Evaluation of Characteristics of Ferroelectric Thin-Film Device]

(1) Evaluation of Wire Bonding Bond Strength

Wire bonding using an Au bonding wire (diameter of 25 μm) was carriedout on the resultant KNN thin-film device, and then the bond strength(tensile strength) between the upper electrode layer and the Au bondingwire was measured. A tensile strength of 60 mN or more was determined tobe “passed” and a tensile strength of less than 60 mN was determined tobe “failed”. Results are shown in Table 1.

TABLE 1 Results of measurement of tensile strength between upperelectrode layer and Au bonding wire. Upper Substrate electrode temper-adhesive ature layer during Type Thick- Upper deposi- Tensile of nesselectrode tion strength Passed/ metal (nm) layer (° C.) (mN) FailedComparative Ti 10 Au No 20 Failed Example 1 heating Comparative 100 55Failed Example 2 Comparative Cr 10 Au 100 50 Failed Example 3Comparative Ni 2 Au 100 55 Failed Example 4 Example 1 10 95 PassedExample 2 50 97 Passed Example 3 Co 10 Au 100 70 Passed Example 4 W 10Au 100 75 Passed Example 5 Mo 10 Au 100 70 Passed

As shown in Table 1, for Comparative Example 1, the tensile strength isinsufficient and then is failed. This is because the substrate was notheated during depositions of the upper electrode adhesive layer 5 andthe upper electrode layer 6, and thus the bondability between the KNNthin-film layer 4 and the upper electrode adhesive layer 5 and thebondability between the upper electrode adhesive layer 5 and the upperelectrode layer 6 would be insufficient.

In the case of Comparative Example 2 where the substrate was heated to100° C. during depositions of the upper electrode adhesive layer and theupper electrode layer, the tensile strength is increased as compared tothat of Comparative Example 1, but does not meet the acceptancecriterion (60 mN). This is because Ti of the upper electrode adhesivelayer 5 would be alloyed with Au of the upper electrode layer 6 duringdeposition to generate an intermetallic compound.

According to Comparative Example 3 in which the type of metal of theupper electrode adhesive layer 5 is Cr, the tensile strength does notmeet the acceptance criterion (60 mN) and is failed. This is because Cris a metal with equivalent or higher easy-oxidability characteristicsthan Ti and thus a Cr oxide would be generated excessively.

According to Comparative Example 4 in which the type of metal of theupper electrode adhesive layer 5 is Ni and the thickness thereof isoutside of the requirement of the invention, the tensile strength doesnot meet the acceptance criterion (60 mN) and is failed. This is becausethe Ni layer is too thin, resulting in that an alloying reaction with Auof the upper electrode layer 6 would be insufficient.

By contrast, it is confirmed that all of Examples 1 to 5 of theinvention satisfy the acceptance criterion (60 mN). It should be notedthat according to a comparison between Examples 1 and 2, it is observedthat the influence of the thickness of the upper electrode adhesivelayer 5 is reduced when the wire bonding bond strength meets theacceptance criterion.

[Evaluation of Ferroelectric Characteristics]

The obtained KNN thin-film devices were measured for the polarizationusing a ferroelectric characteristics analyzer. FIG. 2 is a graphshowing exemplary relationships between polarization value and appliedvoltage with respect to the KNN thin-film devices of Comparative Example2 and Examples 1 and 4.

As shown in FIG. 2, it is found that the polarization property(hysteresis loop of polarization value) of Comparative Example 2 (upperelectrode layer/upper electrode adhesive layer=Au/Ti) is moredeteriorated than that of Example 1 (upper electrode layer/upperelectrode adhesive layer=Au/Ni) and that of Example 4 (upper electrodelayer/upper electrode adhesive layer=Au/W). This is because a Ti oxidewould be excessively generated since Ti is an easy oxidability metal andbecause oxygen deficiency would be occurred in the KNN thin-film layer.In contrast, it is confirmed that the polarization properties ofExamples 1 and 4 according to the invention are almost identical and aresubstantially unchanged.

Regarding the thickness of the upper electrode adhesive layer 5, it wasseparately confirmed that equivalent results were obtained from Examples1 and 2. In addition, regarding the type of metal forming the upperelectrode adhesive layer 5, it was separately confirmed that resultsequivalent to those of Example 1 were obtained from Examples 3 to 5.

The above embodiments and examples of the invention as well as theappended claims and figures show multiple characterizing features of theinvention in specific combinations. The skilled person will easily beable to consider further combinations or sub-combinations of thesefeatures in order to adapt the invention as defined in the claims to hisspecific needs.

LEGEND

-   -   1 . . . substrate;    -   2 . . . lower electrode adhesive layer;    -   3 . . . lower electrode layer;    -   4 . . . ferroelectric thin-film layer;    -   5 . . . upper electrode adhesive layer;    -   6 . . . upper electrode layer; and    -   10 . . . ferroelectric thin-film laminated substrate.

1. A ferroelectric thin-film laminated substrate, comprising asubstrate, and further comprising a lower electrode layer, aferroelectric thin-film layer, an upper electrode adhesive layer, and anupper electrode layer being sequentially stacked on the substrate,wherein: the lower electrode layer is made of platinum or a platinumalloy; the ferroelectric thin-film layer is made of a sodium potassiumniobate (typical chemical formula of (K_(1-x)Na_(x))NbO₃, 0.4≤x≤0.7);the upper electrode layer is made of gold; the upper electrode adhesivelayer is made of a metal that has less oxidizability than titanium andcan make a solid solution alloy without generating an intermetalliccompound with gold; and a part of the upper electrode adhesive layer anda part of the upper electrode layer are alloyed.
 2. The ferroelectricthin-film laminated substrate according to claim 1, wherein the upperelectrode adhesive layer is made of one selected from nickel, cobalt,tungsten, and molybdenum.
 3. The ferroelectric thin-film laminatedsubstrate according to claim 1, wherein the upper electrode adhesivelayer has a thickness of 5 nm or more but 50 nm or less.
 4. Theferroelectric thin-film laminated substrate according to claim 1,wherein: a lower electrode adhesive layer is further stacked between thesubstrate and the lower electrode layer; and the lower electrodeadhesive layer is made of titanium and/or a titanium oxide.
 5. Theferroelectric thin-film laminated substrate according to claim 1,wherein: the lower electrode layer has a main surface with a (111)crystal plane preferential orientation; and the ferroelectric thin-filmlayer has a crystal system of a pseudo cubic system or a tetragonalsystem, and has a main surface with a (011) crystal plane preferentialorientation.
 6. The ferroelectric thin-film laminated substrateaccording to claim 1, wherein the substrate is a silicon substratehaving a thermally oxidized film on its surface.
 7. A ferroelectricthin-film device using the ferroelectric thin-film laminated substrateaccording to claim
 1. 8. A manufacturing method of the ferroelectricthin-film laminated substrate according to claim 1, the manufacturingmethod comprising: a lower electrode layer formation step of forming thelower electrode layer on the substrate; a ferroelectric thin-film layerformation step of forming the ferroelectric thin-film layer on the lowerelectrode layer; an upper electrode adhesive layer formation step offorming the upper electrode adhesive layer on the ferroelectricthin-film layer; and an upper electrode layer formation step of formingthe upper electrode layer on the upper electrode adhesive layer, whereinthe upper electrode layer formation step includes a process ofdepositing the upper electrode layer under a temperature environment of50° C. or more but 200° C. or less to alloy a part of the upperelectrode adhesive layer with a part of the upper electrode layer.